1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine that is equipped with a variable working angle mechanism that changes the working angle of an intake valve.
2. Description of Related Art
As a mechanism that is mounted on an internal combustion engine of a vehicle or the like, there is known a variable valve characteristic mechanism that changes the valve characteristic of an intake valve. Examples of the variable valve characteristic mechanism include a variable working angle mechanism that changes the working angle of an intake valve, a variable valve timing mechanism that changes the valve timing of an intake valve, and the like. For example, in Japanese Patent Application Publication No. 05-001604 (JP-05-001604 A), there is described a mechanism that is equipped with two cams, namely, a large cam with a large working angle and a small cam with a small working angle, and changes over the cam for driving an intake valve between the large cam and the small cam to change the working angle of the intake valve.
In an internal combustion engine that is equipped with a variable working angle mechanism, the working angle is usually changed in accordance with the change in an engine operation state such as an engine rotational speed or an engine load. However, under certain circumstances, the working angle is changed independently of the change in the engine operation state.
An example in which the working angle is changed independently of the change in the engine operation state will be illustrated below. An exemplary case of an internal combustion engine that is equipped with two variable valve characteristic mechanisms, namely, a hydraulic variable valve timing mechanism that changes the valve timing of an intake valve through an oil pressure, and a variable working angle mechanism that changes the working angle of an intake valve through an oil pressure will now be described.
In this internal combustion engine, in some cases, a problem arises in a hydraulic system that supplies a working fluid to the hydraulic variable valve timing mechanism, and the working fluid pressure of the hydraulic variable valve timing mechanism becomes deficient. When the working fluid pressure becomes deficient, the valve timing cannot be held against a cam torque applied to a cam shaft, and the valve timing of the intake valve changes toward a retardation side (FIG. 8A→FIG. 8B). If it is assumed herein that the working angle of the intake valve at that time has been reduced, the valve-opening timing of the intake valve may be drastically retarded from an intake top dead center due to the retardation of the valve timing, as shown in FIG. 8B. It should be noted that “Ex OPEN” and “Ex CLOSED” in FIGS. 8A, 8B and 8C denote the timings for opening and closing an exhaust valve respectively, and that “In OPEN” and “In CLOSED” in FIGS. 8A, 8B and 8C denote the timings for opening and closing the intake valve respectively.
In this case, during a period A from the intake top dead center to the opening of the intake valve, a piston descends to increase the volume of a cylinder while the cylinder remains sealed. Then, when this period A lengthens, the pressure loss of the internal combustion engine becomes excessive to cause misfire. Thus, in such a case, as shown in FIG. 8C, the working angle of the intake valve is increased to advance the timing for opening the intake valve, so that the pressure loss of the intake valve is restrained from increasing to avoid misfire.
However, when the working angle is increased with the valve timing retarded, the timing for closing the intake valve becomes later than an intake bottom dead center. During a period B from the intake bottom dead center to the closing of the intake valve, the air in the cylinder is pushed back into an intake port as the piston ascends. Thus, when the working angle increases by a certain degree at this time, the amount of the air with which the cylinder is filled (a cylinder filling air amount) may become smaller than before the working angle increases. Besides, the working angle must be swiftly increased at this time to avoid misfire. Thus, when the working angle is increased by a certain degree at this time, the cylinder filling air amount, namely, the engine load may be abruptly reduced to cause an abrupt fall in the engine rotational speed.
It should be noted that when the working angle is changed by a certain degree, the engine load may increase to cause the engine rotational speed to rev up abruptly. For example, in the case where the working angle is increased within such a range that the valve-closing timing of the intake valve is earlier than the intake bottom dead center, the engine load increases due to the increase in the working angle, and the engine rotational speed revs up.